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Structure and properties of glasses with a low amount of SiO2 in a quaternary system of Al2O3-SiO2-CaO-MgO
Thermochimica Acta, 194 (1992) 165-173 Elsevier Science Publishers B.V., Amsterdam
165
Structure and properties of glasses with a low amount of SiO, in a quaternary system of Al,O,-SiO,-CaO-MgO C. Oprea a, D. Togan a and C. Popescu b a Institutul Cerceta’ti gi Proiectiiri Tehnologice pentru
Sticki $ Ceramic2 FiGi, Bd. Th. Pallady 305, Bucharest (Romania) b Cent& Cerceta’n’ Calitate qi Protectia Mediului, Str. Siret 95, Bucharest (Romania)
(Received 12 April 1991)
Abstract This study concerns the determination of the compositions of glasses in the quaternary system Al,O,-SiO,-CaO-MgO for SiO, concentrations ranging from 4 to 10 mol.%, and the investigation of their structure and properties. The selected compositions were characterised by melting temperatures below 1500 ’ C. The glasses obtained were investigated by X-ray diffraction, thermal analysis and IR spectroscopy in order to determine their structures and some important properties, namely, refractive index, density, glass transition temperature and crystallisation temperature.
INTRODUCIION
Studies in the field of glass formation for systems like CaO-Al,O,-SiO, and CaO-MgO-Al,O,-SiO, have shown that aluminium ions may be considered as formators if they are found in a tetrahedral coordination [l]. Data published elsewhere [2] show that glasses from calcium aluminate, because of their high tendency to crystallise, can be obtained in small amounts by fast cooling. Larger amounts may be obtained only if the initial composition contains a small amount of SiO,. Glasses made from aluminates are known to possess useful and interesting properties such as high elasticity and good IR transmittance [3,4]. These glasses can also be used to produce glass fibres (tow). There seems to be a lack of information for quaternary glass systems such as Al,O,-SiO,-CaO-MgO that are obtained from recycled raw materials [5,6]. There is also a lack of data for glasses with low levels of SiO,. This paper presents the results from the investigation of a range of glasses which belong to the quaternary system Al,O,-SiO,-CaO-MgO, with an SiO, molar content of less than 10%. The structure and properties of the glasses investigated are discussed from the point of view of their 0040-6031/92/$05.00
0 1992 - Elsevier Science Publishers B.V. All rights reserved
166 TABLE 1 Molar percentages of the constituents Glass no.
of the glasses
Composition (mol.%)
2 3 4 5 7 8 9 10 11
CaO
MgO
50 56 55 45 58 61 58 55 53
10 10 5 15 8 5 5 8 10
&G, 30 30 30 30 30 30 30 30 30
SiO, 10 4
10 10 4 4 7 7 7
change in composition, the Al,O, content being kept at 30 mol.% and the concentrations of SiO,, CaO and MgO being varied. EXPERIMENTAL
The following raw materials were used to synthesise the glasses: Al,O,, SiO,, CaCO, and MgO. They were melted in an oven with silite bars in platinium crucibles, at a temperature range of 1430-1470 o C and a melting time of 2-4 h. All the compositions were homogeneous. The glasses were subsequently baked; the parameters of this treatment were according to the results obtained from the dilatometric and thermal investigations. The compositions, in molar percentages, of the glasses are given in Table 1. The purity of the raw materials was assessed by spectrographic analysis and by atomic adsorption spectroscopy. The results are given in Table 2. The refractive indices of the glasses were measured at room temperature on Pulfrich equipment using a wavelength corresponding to the D band of sodium. The visible transmission of the glasses was measured with M40,
TABLE 2 The impurities in the raw materials Impurity
SiO,
Fe&
0.013 0.01
Cr203 co304
-
cue NiO MnO,
0.0015 0.0045 0.025
f&G, 0.25 0.1 0.043 0.015 0.0127 0.0187
Mgo 0.007 0.01 0.0043 0.0015 0.008 -
CaCO, 0.007 0.0027 0.0006 0.0005 0.0007
167 TABLE 3 The dependence
of the refractive index on the composition of the glasses
Glass no.
Composition (mol.%)
3 7 8 11 10 9 5 2 4
-4w3
SiO,
CaO
I@0
30 30 30 30 30 30 30 30 30
4 4 4 7 7 7 10 10 10
56 58 61 53 55 58 45 50 55
10 8 5 10 8 5 15 10 5
Refraction index, n, 1.66954 1.67037 1.67177 1.65994 1.66104 1.66302 1.64930 1.65332 1.65505
61NIR and M80 equipment manufactured by Karl Zeiss Jena, using samples of 2 mm and 1 mm thick. The IR transmission in the range 1600-400 cm-’ was measured with an M80 (Karl Zeiss Jena) instrument, using KBr pellets. The density of glasses was measured by weighing the samples in air and xylene. The crystallinity of the glasses was investigated by X-ray diffraction using a URD-6 equipment with 28 in the range 17-47 deg. The thermal analysis of the glasses was performed using a Derivatograph C, with alumina as reference, air atmosphere, platinium crucibles, in a temperature range 20-1000’ C, a heating rate of 10 K min-1 and with a sample grain-size of 56-71 mesh. RESULTS AND DISCUSSION
Refraction index
The glasses investigated have indices within the range 1.65-1.67. The effect of the composition on the refraction index is given in Table 3 and the influence of the CaO and MgO contents on n, may be followed. It can be seen that nD increases when the concentration of CaO increases which suggests that the internal structure of the glasses changes with even small changes in CaO level. The same conclusion may be obtained when two sets of samples with constant levels of Al,O, and MgO are observed, see Table 4. Molar refraction
The coordination state of aluminium ions in the system A120,-SiO,CaO-MgO with 30% Al,O,, was investigated by molecular refraction,
168 TABLE 4 The dependence Glass No.
of the refractive index on the amount of CaO in the glasses Constant concentrations
of MgO and Al,O,
10% MgO, 30% Al,O, 2 11 3 4 9 8
5% MgO, 30% Al,O,
1.65332 1.65994 1.66954 1.65505 1.66302 1.67177
computed by the Lorentz-Lorenz equation [7]. Safford and Silverman [S] have published the results for the contribution of Al,O, when the aluminium ions are in tetrahedral and octahedral coordinations, as 12.3 and 10.5 cm3 mol-‘, respectively. Our results are given in Table 5. The data for Al,O, are computed by subtraction, taking the literature values 7.5 cm3 mol-’ for CaO, 7.43 cm3 mol-’ for SiO, and 4.9 cm3 mol-’ for MgO [9]. The results in Table 5 suggest that the AlO, group in the glasses is deformed to varying extents. Viiible transmission
Although no chromophore had been introduced, all the glasses were differently coloured ranging from yellow to brown. We assume that this is the result of impurities in the raw materials (see Table 2), mainly introduced with alumina. The presence of Fe3+ ions is confirmed by a band at about 28600 cm-’ (for glass no. 9); Cr3+ ions are less evident than Fe3+ ions, being indicated by a change in the slope of the transmission curve in
TABLE 5 Determination
of the molar refraction of Al,O,
Glass no.
Density (g cme3)
Molar refraction of glass (cm3 mol-‘)
Molar refraction of f&O, (cm3 mol-‘)
2 3 4 5 7 8 9 10 11
2.9189 2.9296 2.9186 2.9049 2.9353 2.9237 2.9290 2.9407 2.9117
86.0678 215.5124 174.4533 85.0580 218.6029 221.3655 151.8908 123.1028 124.2674
12.24 12.36 12.21 12.32 12.54 12.66 12.53 12.57 12.49
169
22
18
14
IO x3mm-’
Fig. 1. Visible transmission
curve of glass no. 9.
the range 500-600 nm. Figure 1 also shows an increased absorption beyond the visible range. IR transmission
Figure 2 shows a transmission curve for a 2 mm thick sample recorded in the range 13 000-3000 cm-’ on a Specord 61NIR instrument. There is also a strong absorption close to 3400 cm - ‘, the peak area being influenced by the SiO, content; the smaller the amount of SiO,, the greater the area. IR spectral data
The structure of the glasses was investigated on an M80 instrument, the range 1600-400 cm- ‘. The spectra obtained vary as a function composition, as follows.
in of
Glasses 3, 7 and 8
Figure 3 shows the three spectra obtained. Two strong absorption bands at 700-800 and 420-440 cm-’ respectively may be observed for all three
Fig. 2. IR transmission
curve of glass no. 2.
170
4
1400 1200 lwl em 600 kol cm-’ Fig. 3. IR absorption curves for glasses: no. 3, - - - - - ; no. 7, -;
and no. 8, X-X.
glasses. Glass no. 7 has an additional peak at 1015 cm-‘. Table 6 gives the characteristic frequencies of AlO, coordinated groups as obtained from the literature [lo]. According to the literature, the 1015 cm-’ peak is due to the vibration of Si-O-Al bonds. The band at 760-800 cm-’ is considered to be due to AlO, groups. The relationship between the frequencies of vibration and the coordination number is an approximate one as there are influences from the neighbouring groups, the deformation of coordinated groups and the presence of “condensed” or “isolated” groups [lo]. Glasses 9, 10 and 11
These glasses have absorption bands in the ranges 985-1015, 780-810 and 420-440 cm-‘. The height of the first peak at 1000 cm-’ increases as SiO, increases. Glasses 2, 4 and 5 The first and strongest absorption cm-l.
peak lies in the range of 990-1020
X-ray diffraction
An URD-6 X-ray diffractometer was used to identify the compounds that crystallised during the thermal treatment. All the glass samples were first subjected to thermal analysis and were analysed by X-ray diffracto-
TABLE 6 Bond vibration frequency (cm-‘)
Condensed Isolated
of Al-O
AlO, tetrahedral
AlO, octahedral
900-700 800-650
680-500 530-400
171 TABLE 7 X-ray data for glass no. 11 Line no.
Angle (deg)
d (nm)
I (%o)
Phase
d
1 2 3 4 5 6 7 8 9 10 11
18.06 23.44 27.80 28.57 29.76 33.38 35.06 35.54 36.69 41.20 46.68
0.4907 0.3793 0.3206 0.3121 0.3000 0.2682 0.2557 0.2524 0.2448 0.2189 0.1944
100 17.3 16.3 7.9 31.0 80.2 15.7 11.0 34.7 27.3 22.5
12Ca0*7Al,O,
0.4892 0.3791 0.3704 0.2992 0.2680 0.2556 0.2447 0.2440 0.2350 0.2189 0.1945
(nm)
1 (%I 95 16 25 45 100 18 50 35 10 40 30
metry after the exothermal effect. The dimensions of the basic cells were computed and the influence of the modifier oxides was determined. In all cases, only 12CaO * 7Al,O, was identified: Table 7 gives the results for glass no. 11. Table 8 shows the basic cell dimensions of glass no. 11, calculated using a computer program. The value of a agrees well with the ASTM data for 12CaO - 7Al,O, (a = 1.1982 nm). The same compound was obtained for the other samples although the cell parameters were considerably modified. Sample 8, for example, has a = 1.518023 and sample 4 has a = 1.51002. These values imply a more stratified lattice than was expected for these glasses. Theimul analysis A MOM derivatograph C (Budapest), with air atmosphere, alumina reference, a temperature range of 20-1000 o C and a heating rate of 10 K min-’ was used. The recorded curves (DTG, TG, DTA and T) for glass no.
TABLE 8 Lattice constants
a (run) b (nm) c (nm) (YMegI
Value
Standard dev.
Confidence interval
1.197121 1.197121 1.197121 90 90 90
0.000374 0.000374 0.000374 0 0 0
1.196235-1.198006 1.196235-1.198006 1.196235-1.198006 90.000-90.000 90.000-90.000 90.000-90.000
172
-
DTA 473
673
823
1413 1273Ttl$ /-TG
I‘-' mass loss
Fig. 4. DTA, TG, DTG and T curves for glass no. 7.
7 are shown in Fig. 4. There are endothermal effects due to glass transition at 733 and 779”C, respectively, and an exothermal effect at 899°C accompanied by an increase in weight. The curves recorded for all the other samples have similar characteristics, but the effects have different temperature ranges. The weight increase is about 0.7-1, depending on the composition. It is assumed that the gain in mass is due to the reaction [ll] 12CaO * 7Al,O, + H,O + Ca,,Al,,O,,(OH), Table 9 gives the temperatures of the maximum for the second glass transition effect and for the exothermal effect. The high temperatures of the glass transition indicate that these glasses may be used in a high-temperature environment. The activation energy of the exothermal effect for glass no. 7, at heating rates of $7.5, 10 and 15 K min-’ was calculated [12] to be 25.6 kcal mol-‘.
TABLE 9 Temperatures Glass no. 2 3 4 5 7 8 9 10 11
of the endo- and exothermal effects Endothermal (“0 ~788 760 798 780 779 740 780 743
effect
Exothermal effect (“0 980 968
1000 1000 899 938 928 1000 977
173 CONCLUSIONS
The present work demonstrates the compositional range of the glasses formed in the quaternary system Al,O,-SiO,-CaO-MgO with SiO, levels in the range 4-10 mol.% and with a constant Al,O, composition of 30 mol.%. The change in the quantities of CaO and MgO, when Al,O, and SiO, are kept constant, indicates the influence of these modifier oxides on the properties of the glasses. The results of the analyses suggest that the structure of these glasses is composed of AlO, tetrahedra in which there are Al-0-Si bonds and two AlO, tetrahedra associated with each Ca2+ ion. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
S.K. Chopra and C.A. Taneja, J. Appl. Chem., 15 (1965) 157. M. Yamane and M. Okuyama, J. Non-Cryst. Solids, 52 (1982) 217. G.Y. Onoda and S.D. Brown, J. Am. Ceram. Sot., 53 (1970) 311. J.E. Shelby, J. Am. Ceram. Sot., 68 (1985) 155. L.J. Shelestak, R.A. Chavez and J.D. MacKenzie, J. Non-Cryst. Solids, 27 (1978) 75. W.J. Shelestak, R.A. Chavez and J.D. MacKenzie, J. Non-Cryst. Solids, 27 (1978) 83. G.C. Constantinescu, I. Rogca and M. Negoiu, Inorganic Chemistry, Ed. Tehnicg, Bucharest, 1986 (in Romanian). H.W. Safford and A. Silverman, J. Am. Ceram. Sot., 30 (1947) 203. V. Burdick and D.E. Day, J. Am. Ceram. Sot., 50 (1967) 97. P. Tarte, Spectrochim. Acta Sect. A, 23 (1967) 2127. D. Togan, C. Oprea, C. Popescu and E. Segal, Thermochim. Acta, 149 (1989) 387. C. Popescu and E. Segal, Rev. Roum. Chim., 34 (1989) 567.
165
Structure and properties of glasses with a low amount of SiO, in a quaternary system of Al,O,-SiO,-CaO-MgO C. Oprea a, D. Togan a and C. Popescu b a Institutul Cerceta’ti gi Proiectiiri Tehnologice pentru
Sticki $ Ceramic2 FiGi, Bd. Th. Pallady 305, Bucharest (Romania) b Cent& Cerceta’n’ Calitate qi Protectia Mediului, Str. Siret 95, Bucharest (Romania)
(Received 12 April 1991)
Abstract This study concerns the determination of the compositions of glasses in the quaternary system Al,O,-SiO,-CaO-MgO for SiO, concentrations ranging from 4 to 10 mol.%, and the investigation of their structure and properties. The selected compositions were characterised by melting temperatures below 1500 ’ C. The glasses obtained were investigated by X-ray diffraction, thermal analysis and IR spectroscopy in order to determine their structures and some important properties, namely, refractive index, density, glass transition temperature and crystallisation temperature.
INTRODUCIION
Studies in the field of glass formation for systems like CaO-Al,O,-SiO, and CaO-MgO-Al,O,-SiO, have shown that aluminium ions may be considered as formators if they are found in a tetrahedral coordination [l]. Data published elsewhere [2] show that glasses from calcium aluminate, because of their high tendency to crystallise, can be obtained in small amounts by fast cooling. Larger amounts may be obtained only if the initial composition contains a small amount of SiO,. Glasses made from aluminates are known to possess useful and interesting properties such as high elasticity and good IR transmittance [3,4]. These glasses can also be used to produce glass fibres (tow). There seems to be a lack of information for quaternary glass systems such as Al,O,-SiO,-CaO-MgO that are obtained from recycled raw materials [5,6]. There is also a lack of data for glasses with low levels of SiO,. This paper presents the results from the investigation of a range of glasses which belong to the quaternary system Al,O,-SiO,-CaO-MgO, with an SiO, molar content of less than 10%. The structure and properties of the glasses investigated are discussed from the point of view of their 0040-6031/92/$05.00
0 1992 - Elsevier Science Publishers B.V. All rights reserved
166 TABLE 1 Molar percentages of the constituents Glass no.
of the glasses
Composition (mol.%)
2 3 4 5 7 8 9 10 11
CaO
MgO
50 56 55 45 58 61 58 55 53
10 10 5 15 8 5 5 8 10
&G, 30 30 30 30 30 30 30 30 30
SiO, 10 4
10 10 4 4 7 7 7
change in composition, the Al,O, content being kept at 30 mol.% and the concentrations of SiO,, CaO and MgO being varied. EXPERIMENTAL
The following raw materials were used to synthesise the glasses: Al,O,, SiO,, CaCO, and MgO. They were melted in an oven with silite bars in platinium crucibles, at a temperature range of 1430-1470 o C and a melting time of 2-4 h. All the compositions were homogeneous. The glasses were subsequently baked; the parameters of this treatment were according to the results obtained from the dilatometric and thermal investigations. The compositions, in molar percentages, of the glasses are given in Table 1. The purity of the raw materials was assessed by spectrographic analysis and by atomic adsorption spectroscopy. The results are given in Table 2. The refractive indices of the glasses were measured at room temperature on Pulfrich equipment using a wavelength corresponding to the D band of sodium. The visible transmission of the glasses was measured with M40,
TABLE 2 The impurities in the raw materials Impurity
SiO,
Fe&
0.013 0.01
Cr203 co304
-
cue NiO MnO,
0.0015 0.0045 0.025
f&G, 0.25 0.1 0.043 0.015 0.0127 0.0187
Mgo 0.007 0.01 0.0043 0.0015 0.008 -
CaCO, 0.007 0.0027 0.0006 0.0005 0.0007
167 TABLE 3 The dependence
of the refractive index on the composition of the glasses
Glass no.
Composition (mol.%)
3 7 8 11 10 9 5 2 4
-4w3
SiO,
CaO
I@0
30 30 30 30 30 30 30 30 30
4 4 4 7 7 7 10 10 10
56 58 61 53 55 58 45 50 55
10 8 5 10 8 5 15 10 5
Refraction index, n, 1.66954 1.67037 1.67177 1.65994 1.66104 1.66302 1.64930 1.65332 1.65505
61NIR and M80 equipment manufactured by Karl Zeiss Jena, using samples of 2 mm and 1 mm thick. The IR transmission in the range 1600-400 cm-’ was measured with an M80 (Karl Zeiss Jena) instrument, using KBr pellets. The density of glasses was measured by weighing the samples in air and xylene. The crystallinity of the glasses was investigated by X-ray diffraction using a URD-6 equipment with 28 in the range 17-47 deg. The thermal analysis of the glasses was performed using a Derivatograph C, with alumina as reference, air atmosphere, platinium crucibles, in a temperature range 20-1000’ C, a heating rate of 10 K min-1 and with a sample grain-size of 56-71 mesh. RESULTS AND DISCUSSION
Refraction index
The glasses investigated have indices within the range 1.65-1.67. The effect of the composition on the refraction index is given in Table 3 and the influence of the CaO and MgO contents on n, may be followed. It can be seen that nD increases when the concentration of CaO increases which suggests that the internal structure of the glasses changes with even small changes in CaO level. The same conclusion may be obtained when two sets of samples with constant levels of Al,O, and MgO are observed, see Table 4. Molar refraction
The coordination state of aluminium ions in the system A120,-SiO,CaO-MgO with 30% Al,O,, was investigated by molecular refraction,
168 TABLE 4 The dependence Glass No.
of the refractive index on the amount of CaO in the glasses Constant concentrations
of MgO and Al,O,
10% MgO, 30% Al,O, 2 11 3 4 9 8
5% MgO, 30% Al,O,
1.65332 1.65994 1.66954 1.65505 1.66302 1.67177
computed by the Lorentz-Lorenz equation [7]. Safford and Silverman [S] have published the results for the contribution of Al,O, when the aluminium ions are in tetrahedral and octahedral coordinations, as 12.3 and 10.5 cm3 mol-‘, respectively. Our results are given in Table 5. The data for Al,O, are computed by subtraction, taking the literature values 7.5 cm3 mol-’ for CaO, 7.43 cm3 mol-’ for SiO, and 4.9 cm3 mol-’ for MgO [9]. The results in Table 5 suggest that the AlO, group in the glasses is deformed to varying extents. Viiible transmission
Although no chromophore had been introduced, all the glasses were differently coloured ranging from yellow to brown. We assume that this is the result of impurities in the raw materials (see Table 2), mainly introduced with alumina. The presence of Fe3+ ions is confirmed by a band at about 28600 cm-’ (for glass no. 9); Cr3+ ions are less evident than Fe3+ ions, being indicated by a change in the slope of the transmission curve in
TABLE 5 Determination
of the molar refraction of Al,O,
Glass no.
Density (g cme3)
Molar refraction of glass (cm3 mol-‘)
Molar refraction of f&O, (cm3 mol-‘)
2 3 4 5 7 8 9 10 11
2.9189 2.9296 2.9186 2.9049 2.9353 2.9237 2.9290 2.9407 2.9117
86.0678 215.5124 174.4533 85.0580 218.6029 221.3655 151.8908 123.1028 124.2674
12.24 12.36 12.21 12.32 12.54 12.66 12.53 12.57 12.49
169
22
18
14
IO x3mm-’
Fig. 1. Visible transmission
curve of glass no. 9.
the range 500-600 nm. Figure 1 also shows an increased absorption beyond the visible range. IR transmission
Figure 2 shows a transmission curve for a 2 mm thick sample recorded in the range 13 000-3000 cm-’ on a Specord 61NIR instrument. There is also a strong absorption close to 3400 cm - ‘, the peak area being influenced by the SiO, content; the smaller the amount of SiO,, the greater the area. IR spectral data
The structure of the glasses was investigated on an M80 instrument, the range 1600-400 cm- ‘. The spectra obtained vary as a function composition, as follows.
in of
Glasses 3, 7 and 8
Figure 3 shows the three spectra obtained. Two strong absorption bands at 700-800 and 420-440 cm-’ respectively may be observed for all three
Fig. 2. IR transmission
curve of glass no. 2.
170
4
1400 1200 lwl em 600 kol cm-’ Fig. 3. IR absorption curves for glasses: no. 3, - - - - - ; no. 7, -;
and no. 8, X-X.
glasses. Glass no. 7 has an additional peak at 1015 cm-‘. Table 6 gives the characteristic frequencies of AlO, coordinated groups as obtained from the literature [lo]. According to the literature, the 1015 cm-’ peak is due to the vibration of Si-O-Al bonds. The band at 760-800 cm-’ is considered to be due to AlO, groups. The relationship between the frequencies of vibration and the coordination number is an approximate one as there are influences from the neighbouring groups, the deformation of coordinated groups and the presence of “condensed” or “isolated” groups [lo]. Glasses 9, 10 and 11
These glasses have absorption bands in the ranges 985-1015, 780-810 and 420-440 cm-‘. The height of the first peak at 1000 cm-’ increases as SiO, increases. Glasses 2, 4 and 5 The first and strongest absorption cm-l.
peak lies in the range of 990-1020
X-ray diffraction
An URD-6 X-ray diffractometer was used to identify the compounds that crystallised during the thermal treatment. All the glass samples were first subjected to thermal analysis and were analysed by X-ray diffracto-
TABLE 6 Bond vibration frequency (cm-‘)
Condensed Isolated
of Al-O
AlO, tetrahedral
AlO, octahedral
900-700 800-650
680-500 530-400
171 TABLE 7 X-ray data for glass no. 11 Line no.
Angle (deg)
d (nm)
I (%o)
Phase
d
1 2 3 4 5 6 7 8 9 10 11
18.06 23.44 27.80 28.57 29.76 33.38 35.06 35.54 36.69 41.20 46.68
0.4907 0.3793 0.3206 0.3121 0.3000 0.2682 0.2557 0.2524 0.2448 0.2189 0.1944
100 17.3 16.3 7.9 31.0 80.2 15.7 11.0 34.7 27.3 22.5
12Ca0*7Al,O,
0.4892 0.3791 0.3704 0.2992 0.2680 0.2556 0.2447 0.2440 0.2350 0.2189 0.1945
(nm)
1 (%I 95 16 25 45 100 18 50 35 10 40 30
metry after the exothermal effect. The dimensions of the basic cells were computed and the influence of the modifier oxides was determined. In all cases, only 12CaO * 7Al,O, was identified: Table 7 gives the results for glass no. 11. Table 8 shows the basic cell dimensions of glass no. 11, calculated using a computer program. The value of a agrees well with the ASTM data for 12CaO - 7Al,O, (a = 1.1982 nm). The same compound was obtained for the other samples although the cell parameters were considerably modified. Sample 8, for example, has a = 1.518023 and sample 4 has a = 1.51002. These values imply a more stratified lattice than was expected for these glasses. Theimul analysis A MOM derivatograph C (Budapest), with air atmosphere, alumina reference, a temperature range of 20-1000 o C and a heating rate of 10 K min-’ was used. The recorded curves (DTG, TG, DTA and T) for glass no.
TABLE 8 Lattice constants
a (run) b (nm) c (nm) (YMegI
Value
Standard dev.
Confidence interval
1.197121 1.197121 1.197121 90 90 90
0.000374 0.000374 0.000374 0 0 0
1.196235-1.198006 1.196235-1.198006 1.196235-1.198006 90.000-90.000 90.000-90.000 90.000-90.000
172
-
DTA 473
673
823
1413 1273Ttl$ /-TG
I‘-' mass loss
Fig. 4. DTA, TG, DTG and T curves for glass no. 7.
7 are shown in Fig. 4. There are endothermal effects due to glass transition at 733 and 779”C, respectively, and an exothermal effect at 899°C accompanied by an increase in weight. The curves recorded for all the other samples have similar characteristics, but the effects have different temperature ranges. The weight increase is about 0.7-1, depending on the composition. It is assumed that the gain in mass is due to the reaction [ll] 12CaO * 7Al,O, + H,O + Ca,,Al,,O,,(OH), Table 9 gives the temperatures of the maximum for the second glass transition effect and for the exothermal effect. The high temperatures of the glass transition indicate that these glasses may be used in a high-temperature environment. The activation energy of the exothermal effect for glass no. 7, at heating rates of $7.5, 10 and 15 K min-’ was calculated [12] to be 25.6 kcal mol-‘.
TABLE 9 Temperatures Glass no. 2 3 4 5 7 8 9 10 11
of the endo- and exothermal effects Endothermal (“0 ~788 760 798 780 779 740 780 743
effect
Exothermal effect (“0 980 968
1000 1000 899 938 928 1000 977
173 CONCLUSIONS
The present work demonstrates the compositional range of the glasses formed in the quaternary system Al,O,-SiO,-CaO-MgO with SiO, levels in the range 4-10 mol.% and with a constant Al,O, composition of 30 mol.%. The change in the quantities of CaO and MgO, when Al,O, and SiO, are kept constant, indicates the influence of these modifier oxides on the properties of the glasses. The results of the analyses suggest that the structure of these glasses is composed of AlO, tetrahedra in which there are Al-0-Si bonds and two AlO, tetrahedra associated with each Ca2+ ion. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
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